1. Field of the Invention
The present invention relates to a method and an apparatus for measuring retardation of a birefringent material.
2. Description of the Background Art
A drawn plastic sheet generally exhibits birefringence. As to the same material, it is possible to determine the degree of drawing of a sheet on the basis of the degree of birefringence if the sheet has a constant thickness, while it is possible to determine the thickness if the sheet has a constant degree of drawing. Such a birefringent sheet is applied to a liquid crystal display, for example, and it is necessary to measure birefringence also in this case. In addition to this, birefringence of such a sheet material is measured for various purposes. Further, it may be necessary to measure refractive indices along three axial directions of the sheet material.
Birefringence is expressed in refractive indices of ordinary and extraordinary rays, and the difference therebetween appears as phase difference between the ordinary and extraordinary rays transmitted through a sample. This phase difference is called retardation, which is defined by the product of the difference between the two refractive indices and the thickness of the material. Birefringence of the sheet sample is recognized by measurement of such retardation.
In a conventional method of measuring retardation, a sample is placed between two polarizing plates which are arranged in a parallel or crossed nicol manner, so that either of the polarized plates and the sample is rotated. Changes of light transmitted through the polarizing plates and the sample are recorded to obtain retardation from the results by calculation. Such retardation is observed as phase difference between ordinary and extraordinary rays, which are introduced into the sample in the same phase, outgoing from the sample, and this phase difference is generally expressed as (2n.pi.+.delta.), where n represents a natural number of 0, 1, 2 . . . , which is called an order. This order n is increased as the thickness of the sample is increased. The change width of transmitted light intensity which is obtained by rotating either of the polarizing plates and the sample is varied with the value .delta.. Only this value .delta. is directly obtained through the measurement, and there is no method of directly obtaining the order n.
In general, therefore, two beams having different wavelengths are employed to obtain phase difference .delta. corresponding to retardation for each beam. Then the order n is successively changed as 1, 2, 3 . . . to calculate back retardations corresponding to the respective orders, thereby regarding such an order n as correct that retardations calculated as to the two beams of different wavelengths most coincide with each other. According to this method, however, difference .DELTA. of the phase difference by the two beams of the different wavelengths can be obtained only up to 2.pi., and it is impossible to decide an integer m when the difference is 2.pi.m+.DELTA.', where m represents 1, 2, 3 . . . , and .DELTA.' represents 0 to 2.pi.. In practice, therefore, application of this method is limited to an order n of retardation of about 20, and hence it is impossible to measure a thick sample having a large retardation. Further, there is an apprehension for erroneous decision of the order on the assumption that m=0 even if the order n of retardation is in excess of 20 and the aforementioned integer m is 1, 2, . . .
When the retardation is around n.lambda./2 (.lambda.: measuring wavelength), the phase difference is around n.pi. to deteriorate resolution, and it is difficult to correctly decide the retardation value.